The present invention relates to a numerical control apparatus having a superimposing control function for conducting a superimposing X-axis control of a numerical control lathe.
FIG. 1 shows an arrangement of components of a conventional 4-axis numerical control lathe. A work 1 is held by a spindle 2 which is adapted to be rotated about an axis. A tool holder 3 is adapted to be controlled to move both in the directions of X1- and Z1-axes to enable a tool 5 on the tool holder 3 to machine a work 1. Another tool holder 4 is controlled for movement both in the directions of X2- and Z2-axes to enable a tool 6 carried by the tool holder 4 to machine the work 1.
A numerical control lathe has been developed recently, which has a mechanical construction as shown in FIG. 2. This modern numerical control lathe is different from the conventional numerical control lathe of FIG. 1 in that it has an additional spindle 7 and in that the spindle 2 is controllable in the direction of the axis of rotation (Z1-axis direction), while the tool holder 3 is controllable only in the direction of the X1-axis. Since the spindle 2 is movable, advantages are brought about such as possibility of direct delivery of a work 1 on the spindle 2 to another spindle 7 which opposes the spindle 2.
The mechanical arrangement shown in FIG. 2, however, suffers from the following problem. Namely, the movement of the spindle 2 in the direction of the Z1-axis during machining of the work 1 by the tool 6 on the tool holder 4 requires that the movement of the tool holder 4 in the direction of the Z2-axis has to be conducted while taking into account the amount of movement of the spindle 2 in the direction of the Z1-axis. That is, the movement of the tool holder 4 in the direction of the Z2-axis has to be controlled in superimposition to the control of the spindle 2 in the direction of the Z1-axis.
A description will be now given of a conventional art for conducting superimposing control on the Z-axis, with reference to FIG. 3.
A machining program read from a machining program storage section 10 is interpreted by a program interpreting section 11 and the interpreted program is sent to a Z1/X1-axes computing section 12 and a Z2/X2-axes computing section 13. Each of the computing sections 12 and 13 conducts a computation for the position control so that position commands for the X1-axis, the Z1-axis and the X2-axis are delivered to an X1-axis servo control section 14, a Z1-axis servo control section 15 and an X2-axis servo control section 16, respectively. The position command PS delivered to the Z1-axis servo control section 15 is sent also to a variance computing section 26 which computes the variance VA of the command value PS. The variance VA thus computed is delivered to an adding section 27. The adding section 27 then operates to add the variance VA to the Z-axis position command PS derived from the Z2/X2-axes computing section 13, and delivers the addition data AD representing the result of the addition to the Z1-axis servo control section 17. Upon receipt of these position commands, the servo control sections 14 to 17 drive the servo motors 18 to 21 so as to effect the control of the positions in the directions of the respective axes with feedback of position data from position detectors 22 to 25 which are respectively connected to the servo motors 18 to 21.
Thus, in the conventional superimposing control, the variance VA in the movement command PS of the spindle 2 in the Z1-axis direction is added to the Z2-axis position command PC. In this control method, controls of the servo systems for the movement in the Z1- and Z2-axes are conducted completely independent from each other. This poses the following problem. Namely, if the movement in the Z1-axis could not be conducted correctly due to any abnormality occurring in the Z1-axis servo control, the movement in the Z2-axis direction also is conducted in a wrong manner with respect to the work 1, since no means are provided for enabling the Z2-axis control system to recognize the abnormality occurring in the Z1-axis servo control, causing a risk of triggering a serious accident.
Thus, in the control system of a numerical control lathe of the type in which a spindle moves in the Z1-axis direction and a first tool holder moves only in X1-axis direction while a second tool holder moves both in X2- and Z2-axis directions, when a machining is conducted with the tool on the second tool holder during movement of the spindle, it is necessary to conduct a superimposing control for determining the movement of the second tool holder in the Z2-axis direction taking into account the movement of the spindle in the Z1-axis direction. Conventionally, this superimposing control is conducted by using position commands, i.e., by adding the variation in the Z1-axis movement command to the position command of the Z2-axis movement, so that the movement of the second tool holder in the Z2-axis direction becomes out of control in the event of occurrence of abnormality in the Z1-axis servo control system, failing to correctly machine the work.